When one hard steel ball collides with another, kinetic energy is conserved, even if the balls have
different diameters. Why is kinetic energy conserved in such a collision, given that kinetic energy
is not conserved when two unequal length steel springs or rods collide? Experimental results with
bouncing balls, springs, and rods are presented, which reveal the answer. For colliding springs and
rods a significant fraction of the initial kinetic energy is retained after the collision as vibrational
energy in the longer spring and rod. When two hard balls collide, a negligible fraction of the initial
energy is converted to vibrational energy because the collision time is much longer than the transit
time of an acoustic wave across each ball due to the fact that the contact region of a hard spherical
ball is much softer than the rest of the ball.